Free-machining austenitic stainless steel

ABSTRACT

A free-machining austenitic stainless steel and machined products thereof comprising, by weight percent, 0.035% maximum carbon, 17 to 19% chromium, 8 to 10% nickel, 0.5% maximum manganese, 0.25 to 0.65% sulfur, up to 1% silicon, up to 0.20% phosphorus, up to 0.6% molybenum, and balance substantially iron. In the passivated condition the steel is resistant to corrosion, by reason of the critically low manganese content, in environments containing dilute citric, acetic, phosphoric and/or carbonic acids. The combination of machinability and corrosion resistance provides utility for machined fittings used in the beverage and food industry.

United States Patent Denhard, Jr. et al.

Sept. 2, 1975 FREE-MACHINING AUSTENITIC STAINLESS STEEL Inventors: Elbert E. Denhard, Jr., Towson;

William C. Clarke, Jr., Baltimore; Robert M. Larrimore, Jr., Sykesville, all of Md.

Assignee: Armco Steel Corporation,

Middletown, Ohio Filed: Nov. 8, 1973 Appl. No.: 414,141

US. Cl 75/128 P; 75/128 W Int. Cl. C22C 38/44; C22C 38/60 Field of Search 148/38; 75/128 P, 128 W,

References Cited UNITED STATES PATENTS 6/1934 Palmer 75/128 P 5/1957 Prar 75/128 P 3,645,722 2/1972 Aulenbach ct a1. 75/128 P Primary E'\'aminerL. Dewayne Rutledge Assistant E.\'aminerArthur J. Steiner Attorney, Agent, or Firm-Melville, Strasser, Foster &

Hoffman 5 7 ABSTRACT A free-machining austenitic stainless steel and machined products thereof comprising, by weight percent, 0.035% maximum carbon, 17 to 19% chromium, 8 to 10% nickel, 0.5% maximum manganese, 0.25 to 0.65% sulfur, up to 1% silicon, up to 0.20% phospho- 5 Claims, No Drawings FREE-MACHINING AUSTENITIC STAINLESS STEEL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a free-machining austenitic stainless steel which in the passivated condition has excellent resistance to corrosion in environments con taining dilute citric. acetic. phosphoric and/or carbonic acids. The steel of the invention has particular utility for fabrication into fittings used in the processing, packaging and/or dispensing of beverage and food products containing dilute solutions of the above acids.

2. Description of the Prior Art AISI Type 303 is a widely-used free-machining austenitic stainless steel which, in common with some other free-machining stainless steels, obtains good machining properties by reason of the formation of manganese sulfide particles dispersed uniformly throughout the steel. The melting specifications of Type 303 are as follows:

Carbon 0.l5'/1 maximum Manganese 100% maximum Phosphorus 0.20% maximum Sulfur 0.l5/( minimum Silicon L000? maximum Chromium l77( to l9'/1 Nickel 8'7! to [0% Molybdenum 0.6'7: maximum Iron remainder The corrosion resistance of Type 303 is usually adequate in mildly corrosive environments. However, in environments containing dilute citric, acetic. phosphoric and/or carbonic acids. it has been found that themanganese sulfide particles of the steel are vigorously attacked and dissolved by these acids, and that as a result of this sulfide dissolution, pitting or localized c'orrosion of the steel matrix adjacent the manganese sulfide particles is initiated, thereby effecting the release of both sulfide and iron into solutions in contact therewith. It is evident that such contamination could not be tolerated in beverage and food products such as fruitflavored soft drink syrups.

AISI Type 304 is a widely-accepted austenitic stainless steel which possesses adequate corrosion resis tance under a wide variety of conditions, including en vironments containing citric, acetic. phosphoric and/or carbonic acids. However. Type 304 has poor machinability.

It is thus apparent that Type 303 stainless steel possesses good machinability but has poor resistance against localized and general corrosion in environments containing dilute citric. acetic, phosphoric and- /or carbonic acids, while Type 304 possesses sufficient corrosion resistance in such environments but has poor machining characteristics. Since fittings such as valves, connectors, and the like used in the beverage and food industry would be prohibitive in cost if not made from stainless steels possessing good machinability, it is evident that there is a definite need for a stainless steel which provides the combination of good machining characteristics and resistance to attack in environments containing dilute citric, acetic. phosphoric and/or carbonic acids.

In an attempt to maintain good machinability without sacrificing corrosion resistance, the prior art has suggested the substitution of chromium sulfides in place of manganese sulfides in ferritic stainless steels, by reducing the manganese content of the steel. It has been reported that a dispersion of chromium-rich sulfides throughout the steel resulted in adequate machining.

characteristics. although inferior to manganese sulfide in this respect. An article by H. W. Garvin and R. M. Larrimore, Jr. in Mechanical Working of Steel ll. Volume 26. pages 133-144 (1965), reported that the addition of sulfur to a ferritic 177r chromium stainless steel resulted in the formation predominantlyof chromium sulfide when the manganese content was below about 0.40% by weight.

The substitution of chromium sulfide in place of manganese sulfides has thus been suggested in ferritic sulfur-treated stainless steels, and the prior art has indicated that adequate machining characteristics are retained in such steels. However, to the best of applicants knowledge the substitution of chromium sulfide in place of manganese sulfide in an austenitic sulfur treated stainless steel such AISI Type 303 has not previously been suggested or considered by the prior art.

SUMMARY It is a principal object of the present invention to provide an austenitic stainless steel having in combination excellent machining characteristics, and excellent resistance to corrosion in environments containing dilute citric, acetic. phosphoric and/or carbonic acids.

The present invention constitutes a discovery that an austenitic stainless steel. containing sulfur in an amount sufficient to combine with chromium can be made corrosion resistant in acid environments of the abovementioned type provided a critically low manganese content is observed. and further that a critically low carbon content unexpectedly results in excellent machinability. A maximum of 0.035% carbon and a maximum of 0.5% manganese must be observed, with the manganese to sulfur ratio being maintained at less than 2: I.

In its broad composition range, the steel of this invention consists essentially of the following elements, by weight percent:

carbon up to 0.03571 chromium about l7/1 to about 19% nickel about RJ'r' to about l0/.'

manganese up to 0.5%

sulfur about 0.25% to about 0.671 silicon up to about l rl phosphorus up to about 0.20"

molybdenum iron up to about 0.0 remainder. except for incidental impurities At least about 17% chromium is required for general corrosion resistance. More than about 19% chromium provides little additional benefit from the standpoint of corrosion resistance and would upset the austcnitic balweight. with the ranges of other elements remaining the same. Similarly. manganese is preferably restricted to a maximum of about 0.4)? by weight. with the ranges of other elements remaining the same. Sulfur is preferaance of the steel. 5 bly present in amounts of about 0.4 to about 0.5%. with At least 8% nickel is required for general corrosion h ranges f h Clements remaining h Sauna i resistance and for insuring an austenitic structure. mum propcrtics bt i d h n h prcferrgd li it More than about 1071 nickel confers little additional f arbon, manganese and sulfur are ob e d i corrosion resistance and unduly increases the cost of bi mi i h a maximum munguncsc t lf i ht the alloy. 10 ratio of l: l.

A maximum of 0.50% manganese must be observed in order to insure that sulfur is predominantly in the A more preferred composition range for the steel of form of chromium sulfide. Although a small amount of the invention thus is as f ll a" percentages being by manganese sulfide may be present in the steel at the ls weight: maximum content of 0.5% manganese, the amount so combined is too small to affect adversely the corrosion resistance in environments containing dilute citric, i P to about 0-0371 chromium about W71 to about 19% acetic, phosphoric and/or carbonic acids. nickel about to about (7,

At least 0.25% by weight sulfur must be present in so mlmsuncfic "P w sulfur about 0.4% to about 0.5% order to combine with chromium and impart the de- 5mm, up to hum 1 sired freemachining characteristics. More than about pia p B 8 29 t mi'tnm 11 t./ 0.60% sultur provides little additional benefit in mai L u fg ff j f Cerium chinability and adversely affects the hot and cold workincilkmfll impurities ing of the steel.

Molybdenum is not necessary in the steel of the in- The compositions of a steel in accordance with the vention and ordinarily Wlll not exceed residual U t O 67 1 bd L t h t present invention is set forth below in Table 1 together 2 m a'vrn "noun 8 p mo y cm} m L p u with that of a low carbon AIS] Type 303 steel. Corromolybdenum in excess of this amount should be g i d d t w sion resistance tests in beverage syrups were conducted fs m or O mmlmlze f' i on these two steels and are summarized in Table ll be- Sihcon and phosphorus. which are normally present low as impurities. may be tolerated in amounts up to about 171 and about 0.20%. respectively, without adverse eff An empirical test for corrosion resistance was deect on the properties of the steel. The remainder is vised and carried out with the steel of the present Insubstantially iron. except tor incidental impurities usu H d th th d t, f t l t l vention and with the AlSl Type 303 steel having the H g/$ 1 W1 pm :2 5 l f h a compositions set forth in Table l. The test environ- 3' t F g g O L t mvLn} ments were syrups of soft drinks sold under the registion falls within the roa melting spcci ications o tercd trademarks COCA COLA and FRESCA respec AISl Type 303, the critically low carbon and manga- 4O tivcly Corrosion resistance was determined by the Contents of the Prescnt p result amount of iron picked up in the syrups after immersion excellent resistance against corrosion in environments f a Sample in syrup (proportioned so as to provide 1 containing dilute citric. acetic, phosphoric and/or carbi i h f syrup per 20 Squaw in h of m l for bonic acids. and in excellent machining characteristics 96 hours at room temper-guru F lL h Syrups which are superior to those of AlSl Type 303. It should were used as test solutions. Diluted values were calcube noted that AlSI Type 303 stainless steel sold comlated by dividing iron pick-up by 6.5. mercially in this country contains from about 0.06 to about 0.1% carbon and from about 1.25 to about 1.5% A i di d i T bl [1 h steels were tested i h manganese. as-machined condition. and with two different surface assivatin treatments. Such assivatin treatments are DESCRIPTION OF THE PREFERRED p g P g well-known in the art and are commonly used to clean EMBODIMENTS H machined stainless steel parts generally and to remove Within the above broad composition range. carbon is iron particles which may be embedded in the surface of preferably restricted to a maximum of about 0.03% by the part as a result of machining.

TABLE I COMPOSITIONS IN WEIGHT PERCENT Type or Steel (r Ni Mn 5 si P Mo Present Invention .03] [7.59 9.37 .40 .34 .6] .020 28 Low Carbon A15! 303 .031 mu 9.30 1.34 .33 .04 .022 2a TABLE ll (ORROSlON TESTS IN BEVERAGE SYRUPS lMMERSlON lN SYRUP 96 HOURS AT ROOM TEMPERATURE Test Syrup FRESCA Fe in ppm (Average of 2 Samples) Full Strength Test Syrup ((XA-(OLA Fe in ppm (Average of 2 Samples) FullStrength (ontlition Type of Steel Diluted Syrup Syrup Diluted Syrup Syrup Present invention 3.3 1.4 2.5 In: Ass-machined AlSl 303 3.2 20.8 2.0 16.9 10% HNO;,5"i Na C- Present Invention 0.90 5.85 0.60 3.90 r +6 HCl+20/i HNO" AlSl 303 l. l 7. l l.(\ [0.4 S'd Na. ,Cr. .O l0'/(HNO;,3'/;HF+ Present lm'ention 0.58 3.77 0.19 l -3 ZUAHNO- SJNa C- AIS] 303 0.90 5.85 0.86 X

The test results of Table ll show that the steel of the present invention exhibits resistance to attack by beverage syrups superior to that of AlSI Type 303 steel. The relatively high values reported for the as-machined condition result from iron particles embedded in the surface as a result of machining and can be disregarded. Table [I further demonstrates that optimum corrosion resistance in the steel of the invention is achieved after a surface treatment which involves a short exposure to dilute nitric and hydroflouric acids followed by passivation treatment in nitric acid containing sodium dichromate. The iron pick-up of the steel of the invention in this condition in COCA-COLA test syrup was exceptionally low.

It is therefore apparent that low manganese is essential in obtaining the excellent corrosion resistance of the steel of the invention.

Machinability tests have shown that the steel of the invention exhibits machining characteristics superior to those of regular AlSI Type 303, Le, with a carbon content of 0.06 to 0.1% and a manganese content of 1.25 to l.5%. Tests were conducted by making successive 1/16 inch transverse cuts on approximately 1 inch diameter bar (outside to center) with a constant feed of 0.0025 inch per revolution using high speed tooling and sulfur-based oil. The highest surface feet per minute (SFM) for a 5-hour cutting tool life was taken as the machinability index of the alloy. The results, reported as an average of at least 5 samples of Type 303 from different sources with carbon contents ranging from 0.06 to 0.1%, and the steel of the invention of Table I, were as follows:

5-hour form tool life SFM AlSl 303 (0.U(i/?-0.l'/ C) Present Invention (composition olTable l) The steel of the invention may be melted, cast, hot reduced and cold reduced to plate, sheet, strip, bar and rod by conventional steel making procedures. The hot working characteristics, cold forming qualities and mechanical properties are equivalent to those of regular AlSl Type 303. The steel of the invention. after conventional processing into plate, sheet. strip, bar or rod form, is then readily fabricated on conventional machines into parts and fittings, such as valves, tubing, containers and the like, after which the surfaces of the machined fittings are subjected to passivating treatments of the type disclosed above or other types conventional in the art. The steel can be subjected to an nealing treatments of the type generally applied during hot and cold reduction without adverse effect. Good welding characteristics are exhibited by the steel of the invention both in the hot reduced and cold reduced and annealed conditions.

It is known in the art that the size, distribution and composition of sulfide inclusions are important in determining the machinability of free-machining steels. More specifically, machinability increases with increasing sulfur content. increasing sulfide size and decreasing length to width ratio of the sulfide inclusions. However, high sulfur contents in excess of about 0.65% result in inferior hot workability and decrease in transverse strength and toughness. Although forming no part of the present invention, such conventional prior art techniques in obtaining optimum size and distribution of sulfide inclusions are preferably followed in order to achieve optimum machining characteristics.

The invention has been disclosed by way of preferred exemplary embodiments. but it will be apparent that modifications may be made without departing from the scope of the invention. Hence no limitations are to be inferred except insofar as specifically set forth in the claims which follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An austcnitic, free-machining stainless steel having excellent resistance to corrosion in environments chosen from the group consisting of citric, acetic, phosphoric, carbonic acids and mixtures thereof, and excellent machinability, said steel consisting essentially of, by weight percent, up to 0.035% carbon, from about 17 to about 19% chromium, from about 8 to about l0% nickel, up to 0.5% manganese, from about 0.25 to about 0.6% sulfur, up to about 1% silicon, up to about 0.20% phosphorus, up to about 0.6% molybdenum, and remainder iron except for incidental impurities, the manganese to sulfur weight ratio being less than about 2:], whereby sulfur is predominantly in the form of chromium sulfide.

2. The steel claimed in claim 1, containing a maxi mum of about 0.03% carbon, a maximum of about 0.4% manganese, and from about 0.4 to about 0.5% sulfur.

3. The steel claimed in claim 1, wherein the maximum carbon content is about 0.03%.

4. The steel claimed in claim 1, wherein the maximum manganese content is about 0.4%. and wherein the maximum manganese to sulfur weight ratio is 1:1.

5. The steel claimed in claim 1, wherein sulfur is from about 0.4 to about 0.5% and wherein the maximum manganese to sulfur weight ratio is l:l.

=l= l l l 

2. The steel claimed in claim 1, containing a maximum of about 0.03% carbon, a maximum of about 0.4% manganese, and from about 0.4 to about 0.5% sulfur.
 3. The steel claimed in claim 1, wherein the maximum carbon content is about 0.03%.
 4. The steel claimed in claim 1, wherein the maximum manganese content is about 0.4%, and wherein the maximum manganese to sulfur weight ratio is 1:1.
 5. The steel claimed in claim 1, wherein sulfur is from about 0.4 to about 0.5%, and wherein the maximum manganese to sulfur weight ratio is 1:1. 